Method of controlling rectifiers



June 22, 1937. w. H. HOWE R 20,418v

I I METHOD OF CONTROLLING RECTIFIERS AND CIRCUITS THEREFOR OriginalFiled July 25, 1933 10 Sheets-Sheet 1 0 mam); W a -2 -1 05 i-w I w (if W135' W I ZZ- 'JNVENTOR.

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METHOD OF CONTROLLING RECTIFIERS AND CIRCUITS IIIEREFOR Original Fil edJuly 25, 1933 10 Sheets-Sheet 3 A TTORNEYS.

June 22, 1937; w.- H. HOWE manor) 0]? CONTROLLING macnmmis min cmcun'sTHEREFOR I Original Filed July 25, 1933 10 Sheets-Sheet 4' WaZZr INENIdIL flzji'ai fl (7% s ZATTORNEYS.

June 22, 1937.

w. H. HOWE METHOD OF CONTROLLING RBCTIFIERS AND CIRCUITS THEREFbROriginal Filed July 25, 1935 1Q Sheets- Sha n 5 V I I IN VEVTOR 2 ll BYM mm M ATTORNEYS.

June 22, 1937. I w. H. HOWE 20,418

METHOD OF CONTROLLING RECTIFIERS ANT) CIRCUITS THEREFOR Original FiledJuly 25, 1933 10 Sheets-Sheet 6 ATTORNEYS.

I Ju1 1'e '22, 1937. w. H. HOWE 20,418

METHOD OF CONTROLLING RECTIFIERS AND CIRCUITS THEREFOR Originai FiledJuly 25, 1933 10 Sheets-Sheet 'T IN I EN TOR.

, aMZ/Zazm W MM ATTORNEYS,

' June 22,1931. w. HjHo E R 2 .418.

METHOD OF CONTROLLING RECTIFIEHS M ID CIRCUITS THEREFOR Original FiledJuly 25, 1955 1o Sheets-Sheet a or w- 1/ f .12

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June 22,1937. w. H. HOWE Re; 20

METHOD OF CONTROLLING RECTIFIERS AND CIRCUITS THEREFOR D. a. deltaATTORNEYS.

Reiuued June 22, 19 37 Wilfred 11mm, Winchester, aasignor to- AtlanticPrecision Instrument Company, Hai- I den, Masa', acorporationol'liasaachuscttl Original No. 2,020,314, dated November 12,19:5,

Serial'No. 682,120, July 25, 1933.

Application Y for reissue November, 12, 1935,,Serlal No. 49,432

' 19 Claims. (or. 112-119) This invention relates to the control ofrectifiers of the arc type with separate control of the ignition timeamong which are those of the gas filled thermionic or hot cathode type,such, for

example, as those containing mercury vapor, or

inertgases such as neon or the like. It is characteristic of theserectifiers that they require a voltage is thereafter made negative to orbeyond a point which would have prevented the. flow of controlledcurrentif it were not, already flowing. Where an alternating potentialcauses the flow of ;controlled current, however, the reversal ofpotential causes the controlled current to cease so that if thecontrolling voltage is thensufliciently negative, controlled currentwill not again flow on the positive half of the cycle. This inventionwill be further described with particular reference to rectifiers of thehotcathode type,- but it should be understood that unless specificallysostated theinvention includes all recti-v fiers utilising, a separateignition time control voltage. I

Heretofore two general methods 'of control have been utilized forrectiflers of this character; In accordance with one methoddirectpotential only is impressed on the grid. Hthisdirectcurrentis morenegative than a given value depend cut on the temperature of the tube"and the'po tential of the output circuit, no'current flows is ,morepositive than this value, the tube is wide open".

40 at current flow-if flow at-all. I The other method consists, inimpressingon the grid an alternating potential oi suitable mg- I nitudein adjustable phase relation to .thealternating potential-."applied tothe" output circuit".

Byv adjusting this phase relation the time of start of the platecurrent'inthefpositive half -cycle of The present inventionlrelates toadifferent method of control having the d ifi i asc of comT I trol oftheaverage output currentjflow possessed controlling the tube.

in the, output circuit, while if the-gridpotential' With this method ofcontrol the tube :acts. as a relay, being either closed or fully" open.This method cannot control the amount conditions are correct for I Imonic frequenc by the phaseshii't method but without requiring variablephase control. Inlaccordance with the method of this invention, thecontrolisprovlded through a variable direct current potential which isoften much easier to obtain than a variable phase alternating controlpotential. For example, any type of vacuum tubecimuit may be used, theoutput from such a circuit being in eflect a variable direct current,and this may be intensified by one or more stages of amplification so asto be adequate both in potential andpower for such control. There aremany other types of circuits also where it is an easy matter to obtain asource of variable direct current potential for With this type 'ofcontrol it is possible by choosing the proper phase relation betweenanecessary alternating current grid potential com ponent ,andthealternating current input to the tube to determine the character orresponse in tube output to a change in potential of the controllingdirect current grid potential.

In order that the application of such variable directcurrent for controlpurposes and the efiects thereon or various phase relations betweenthealternating current grid component and the. alternating currentinput maybe betternnderstood, reference may be had to the accompanying drawingsin which Figures 1 to 6 are diagrams illustrating average values ofcertain characteristicsv of a mercury vapor fllledtube, such a tuberequiring a nega- .tive grid potential to prevent plate-cathode curphaserelation between the alternating current.

grid. component and the-inputvoltage; 3

Flgurell is a diagram developed iromthe dia-- gramsofFigurcs 8, 9 and10. l I

Figures 12, 13, l iian'd- 15 are further showing the 'eflects oi varyingphase relation between alternatinggridjvoltage and the input power. IFigures 16,17 andlB are. further illustrating the eflectsottheintroduction loi' harof grid. potential,

mentfor control of a motor; from rotaies m the alternating componenttionatfull speed in one direction through stop, 7

meme, speed. th posite direction.

9 e msmmmm --Y W. fa W f I relations between the controlling andcontrolled j mechanisms. I

Figure22isawiringdiagramotoneoithe unitsshowninFigur-efll.

Figure23lsadiagramoiatypicalB-Hcurv -i'or a standard grade or laminatediron.

Figure 24 is a diagram showing the magnetizing current flowing throughan inductance having the core of the iron of Figure 23 and on whichinductance is impressed a sine wave electromotive force. 7

Figures 25 and '28 are curves showing two difierent values ofalternating potential added to the harmonic potential of Figure 24.

Figure 27 is a curve illustrating the effect applying the voltage shownin Figures 25 and 26 on the rectifier grid.

Figure 28 is a curve showing the plate current resulting from. the gridvoltage cycles of Figure This application is a continuation in part ofmy application Serial No. 562,694, filed Septemher 14, 1931, for Method01" controlling gas-filled thermionic rectiflers and fundamental circuittherefor.

In Figure 1 are shown two average characteristics of a tube oi thisdescription with the minimum temperatures within the tube of 30 and 60C. and show the values of grid cathode potential which isiust suflicientto prevent ionization within the tube at the anode cathode potentialsshown, theordinates for these curves being plate voltage Ep, and theabscissas representing grid voltage. It is characteristic oi gas filledtubes that as long as the grid potential E is more negative than theval'ueon the curve, anode-cathode current will be very small, the tubethen being closed, while as soon as the grid cathode potential becomesmore positive than this value, a large anode-cathode current flowslimited primarily by the external impedance of the anode-cathodecircuit, the tube then being wide open, and this large value of currentwill continue to flow regardless of the grid potential unless and untilthe potential of the anode-cathode circuit is interrupted ormeVersed.

Figure 2 is derived directly from Figure l. E, represents theinstantaneous plate voltage for a 220 .volt root mean square sinusoidal"potential representing the potential applied to the anode' resent thecorresponding grid cathode potential for minimum tube temperaturesoi! 30and C lmmmely' sumac the tube closed throughout the cycle with this ofthe tube, that is. Just suflicient' to prevent current flow in theanode-cathode 'cir'cuit. At

--any instant iithe actual grid cathode potential has remained and ismore negative than Ec no current will flow in the anode-cathode circuit,and current will commence to. now in the anodecathode circuit at thatinstant when the gridcathode potential becomes more positive than Ec,such current continuing through the remainder of the positive hall ofthe cycle of the anodecathode potential. This curve of. Figure ,2 .isplotted against degrees, the zero point. for de-. area being taken asthe aero-potential'oi the anode-cathode circuit. V

Figure-3 shows the curveslku and E01, potential on grid cathode 'i'o'rcut-oil, taken from Figure 2. curves: are plotted iour some other curves0, b, c and d,'each consisting oi the sum of a sinusoidal alternatingpotential oi 5 volts root mean square in phase lagging the anode-cathodepotential by 45 and a'constant direct current potential of plus 5, zero,minus 5 and minus 10 volts, respectively. From an inspection of thesecurves it is apparent that with I the plus 5 volts direct current (curvea), the grid cathode potential is always positive with respect andwillthen become positive with respect. to

this cut-oil potential. In other words, the tube will be open wide about30 alter the positive anode-cathode potential is applied so that currentwill commence to flow at that time and will continue throughout theremainder oi the positive half of the cycle. Thus the average currentflow will be less than if the tube were wide open during the wholepositive half cycle, as when a direct current potential of plus 5 voltsis superposed on the alternating grid potential as shown by the curvea.If the direct current component of grid-cathode potential is reduced tominus 5 volts as shown by curve 0. the intersection of this curve withthe curve Ee indicates that the current in the anode-cathode circuitwill commence about 60 after the potential is applied.

With a direct current potential oi minus 10 volts, shown in the curve d,the potential of the grid is always more negative than the cut-offpotential: and as a result substantially no current will flow throughthe anode-cathode circuit as the tube will remain closed.

Figure 4 is similar to Figure 3, except that the alternating potentialcomponent of the gridcathode potential is 10 volts root mean square forall curves instead of 5 volts as in Figure 3. Thus with plus 10 voltsdirect current component as shown by the curve e, the tube will open atthebeginning oi. the positive half cycle. With plus 5 4 volts directcurrent it will openabout 15 after the commencement of the positive halfcycle (curve I). With minus 10 volts the tube will open approximatelyafter the start of the positivehalt cycle (curve :i) though with thelower alternating current plate voltage. of Figure 3 the tube will notopen at all with this negative direct current potential. Even theme ofminus 15' volts direct current voltage component will fail to holdhigher value of alternating voltage as is shown in curve It. I

Figure 5 shows the relative phaseposition of the anode-cathode potentialat the start of anode cathode current flow for all values between plusapplied to the grid together with-the same 5 volts root meansquarersinusoidai alternating potential as in Figure 3, this curve intact being in summary of the points of intersection of a complete familyof curves such as the curves a, b, c, and d of Figure 3 with the curvesE01 and Ecz.

. 10 and minus 20 volts of directcurrent potential 1 This figure shows.how, by varying the direct curlimits- Figure 6 is similar to Figure 5,except-that it are similar "in shape, but .t stm o aco'nsiderably largerchange of direct current apparent, therefore, that by adjusting thealternatingpotential, the relation of change oi ur p ential? o the cc tm lens anode-cathode power be set within .wide

The phaselagof the alternating current potential component of the gridcathode voltage has been shown as 45 for, all the curves. tion ofFigures 3 and 4, however, shows that an increase oi! this angle of lagincreasesthe angle' at which the cut-oil. voltage is reached and alsoincreases somewhat the direct current component necessary to malre thetube start to conduct in the anode-cathode circuit as soon astheanodecathode potential is'applied. The amount of this phase lag, as willlaterbe more fully described, determines the character of the outputcontrol. Where itis desired that the main control of the tube bebydirect current grid voltage variations,

in order that smooth, full range control maybe had,the alternatingcurrent grid phasevariation pos'sibleis'strictly limited, as willlaterbe. more fully explained.

former 2 supplying power to an output circuit which is illustratedas themotor at 3, though it with power of any desired frequency-as,- forexthree secondaries. One secondary, l4, energizes the operation of thecontrolling circuit. The, con.-

The first comprises a, source of varying direct current potentialillustrated at 6 from which it is desired to control "the outputcircuit. it will be noted that thissource 6, as shown is entirelyindependent of any circuit controlled by the, tube,

in any way by the tube or itsimmediate essential circuits. As.hereinbefore pointed .out, this: may be a vacuum tube system operatedby anysuitable Allen Patents No. 1,708,374 granted April 9,- 1929, and:No, 1,781,153 granted November '11, v 1930, or a direct current magnetofor speed controL-or any other source from which'it', is desired-toexercise a power. control. .The second part comprises a source of'directcurrent potential of determined valuesuch as a battery :1 and apotentiometer ii. The third comprises a source oi" alternating po-,

is made from the'values shown in Figure 4. It willbenotedthat the curvesof Figures band-,8

tial-i's necessary"for--the same change of time or the anode-'cathode,current.. 'It 'will be Figure 7 is a diagram of the fundamental circuitmaking use of this type of control. ,The gas filled tube indicatedat I is energizedby the transshould be understood that any desired loadmight be substituted, The transformerv ,2 is. supplied ample, 110 voltsat 60 cycles per second and has.

the cathode of the tube. A second, 4, supplies power to the outputcircuit which leads to the,

plate of the tube, while a third, 5, is utilized in trolling circuitconsists of three essential partsor of any other circuit controlling thetube, solthat v the effective value of its potential is not-modifiedsource,-such for example, as a light sensitive tube or a measuringsystem such 'as is shown in the,-

tent'iaiofthe same frequency as that'applied to; the output circuit andso arranged; that any desired'ilxed value-o1 potential in any desiredphase relation to the outputcircuit potential maybe obtainedi third'part will bemore fully" desc'ri'bedi These three parts are,connectedin. serieslbetween'the cathode of thetube and th 'grid.y Theymay ibenix-any 'desire'dforder. As? shownthe variable direct currentcontrolling po I tential is neirt' "to the grid and the direct Iourrent;

of fixed value is adiacentthereto, but i any other order would be assatisfactory. third part, which comprises thesource oi potential; offcontrollable magnitude and phase, is suppliedgby th networlr consisting.oi' the transformer secondary-5, 'a jinduc -tance]!,'a resistor .IIwith tw j taps I l and I2 and a'condenser' ii. The transformersecondarysupplies a ilxed potential-exactly in phase with thepotentiai o1theoutput or plate circuit. This potential causes 'a'current flowthrough theinduc'tance I theresistor "and the condenser l3. Iif thecondenser and. inductance are of equal impedance they exactly balanceout and this cur-, rent isexactlyin phase with the voltage oi. the

transformer secondary 5. The current flowing through thecondenser llproduees an impedance drop 90? lagging the current and hence 90 laggingthe pctential oi. the input circuit. Since the inductance and thecapacity exactly balance out oithe complete circuit the-current flow isdetermined by the resistance l0 and can be set up at any desired valueby adjusting the tap l2. By varying this resistance and thus thecurrent, a potential across the condenser of any desired value beobtsmea'up to the limit 01' the transformer secondary 5... By adjustingthe relative positions of the taps I and .l 2 any desired.alternating'potential lessjthanthis in phase with the output circuitpotential can be obtained. The totalalternating component potential ofthe control circuit is the sum of. the potential across the condenserl3, and that between the taps Ii and i2. Thesetwo potentials, however,are 90 apart in phase and each'may be set at anydesired magnitude sothat theirvector sum may be adjusted to any desired magnitude within thelimits imposed by the potential of the transformer secondary 5 and atany; desired phase between in phase and 90lagging the output'potential.

In practice theconstruction of this-phase magnitude control circuit may:be varied. Forrex-j ample, i'or'motor control circuits where the minimumcurrent in the output circuit should beset fairly large and hence themaximum phase diflerence between the output circuifland, the alter-I-nating component of the controlling circuit is fairly small thecondenser I! may be dispensed With-.1 In practice'also a certain 'valueor range oivalues-of alternating potential for the control.

circuit-may be. necessary or desirable and any circuit providing thispotential in the desired phase relation tothe output circuit may be'used. v Byproperly: adjusting the magnitude and phase a relation of; 1this 3 alternating component of grid voltage, and the negative directcurrent voltage;

changes otthecontrollingdi'rect current voltage J of predeterminedamount maybe caused to' prfo duce the] desired variations in current[flow in tlie. pu 1 m? 1 ,0. j1 "-D. d 1

e 'ii se in utput cu rentflfor'any .varia; tion' in the controllingcircuit. whiiethe con as entirely separate from the negative. c nstant",

stances misht'be represented by, atsingle; variablefcontrolling voltageilnairitained negative direct currentlxvoltage, of course, in'man'yj'm':

with res'pectzto the Zgrid. Likewise the control;

1 r t rs w li ze'm be he S merian or we came:- ec er he" r t" m z s,-each' of which has, its.;,eii.'ect on the controlled.

, sidered, as quite regardless of the-"direct current mains morenegative with respect to the filament grid pgtential and or magnitude ofalternating current grid potential, the phase of the alternating .gridvoltage relative to that of the input must lie within certain limits forcontrol of plate grid which is Just sufiicient to cause the tube to,

open and pass current. So long as the grid rethan the value indicated onthe curve Eu, this flow'of current inthe plate circuit is prevented,

but atthe first instant that the grid becomes less negative with respectto the filament than the value Eu, the flow of plate current willcommence and once started will continue until interrupted by thealternation of the plate voltage at the 180 line.

In Figure 8 it is assumed that conditions of 1 I the grid circuit arecorrect to permit plate current fiow during a small part, about 10,- ofeach cycle. The grid voltage thus reaches the critical value Ee only tendegrees to the left of the 180' line of this figure, or, inother words,the grid permits the plate currentto start flowing 10. before the.reversal of plate potential interrupts this flow. Two combinations ofalternating anddirect potentials are illustrated which accompliah'thiscontrol in the' curves marked A and B. In bothcurves'and' throughoutFigures 8, 9, and 10 a constant magnitude alternating grid potentialwith a peak amplitude of forty-five volts has been used. In Figure a,curve A, this potential has-been set up lagging the plate to filamentpotential by 175 and a direct current component of plus 5 volts usedwith it. It will be noted that if either the angle or lag, or the directcurrent component be materially increased, then this grid p0 tentialcurve will intersect the critical grid volt age line Ec not only at the10 line near to the 180 line, causing the plate'current fiow during 10of the cycle, but will also intersect the critical voltage curve Ec atits left hand end causing the plate current to start at the beginning ofthe cycle. That is, if the alternating current potentia'l-lags bysomewhat more than 175, the tube will open wide or not at all.Furthermore, with the alternating potential la'ggingby 175' the tubecannot be made to open gradually by direct current control through muchmore than 10 of plate current fiow before the tube will open wide. Incurve B, on the other hand, with 'the alternating potential lagging by90 combined with a .direct current potential of minus 45 volts, the

direct current potential is made less than fortyfive volts, that is,becomes more positive, so that the curve Has a whole'is elevated fromthe posi- -tion shown, then gradual increase of plate current flow willoccur. Thus the plate current will start earlier, but this earlier startwill be proportional to the direct, current grid potential. If, however,this alternating current potential is used lagging by less than then itspoint of intersection with the grid voltage curve Es will occur at anearlier point in the cycle so that a decreased direct current willcause, not a gradual decrease of time of plate current flow to acomplete closing, but a sudden complete interruption of all fiow ofcurrent in the plate circuit when the grid some. I "4 plate voltage islowered out of contact with the curve Es whichwill be at a point lessthan 10 oi',plate. current flow, that is, less than 10 from the 180'point where the plate voltage reverses. To summarise the ioregoing, toproduce a plate current fiow during ten degrees of the plate current.cycle, it is n that the controlling grid alternating potentialin thegrid circuit shall lag the in tlre'plate circuit by an angle not lessthan \nor more'than 175 Ii the angle of lag is less than 90', variationorthe direct current component in the grid circuit will cause the tubeto either shut oi! plate current fiow entir lflor to pass plate currentthrough an angle of more thantendegrees. If the angle of lag is morethan-175, then variation of direct current component in the grid circuitwill cause the tube to either permit current flow for less than tendegrees, or else to permit current flow through the entire positive halfof the cycle.

Figures 9 and 10 illustrate the same eifects for plate current fiowduring 90 and respectively. Figure 9 illustrates particularly well thatat the condition oi maximum grid voltage lag (curve A) direct currentpotential control may be exercised-for all values or plate current timeless than 90" while at the condition of minimum grid voltage lag, (curveB) direct current potential control may be exercised for all values ofplate current time more than 90". In Figure 10 where the plate currentfiow is during 170' the minimum ,gridvoltage lag has become less thanzero,

that is, has become lead instead of lag, but the effects are the same.

It'may be noted from these figures that though only a single. value ofmagnitude of alternating grid potential has been shown, the sameconclusions are equally true for all values of alternating gridpotential, the only etfectof variation of ala complete cycle ofthisvoltage the limiting I values'ofj phase or grid'alternating potentialfor control at corresponding points in the cycle. The

most important eifect shown by summary is -that full range smoothcontrol is possible only with the grid potential-lagging at just so"; n

the angle is'less than 90, then as the direct con-J tro current in thegrid circuit becomesgradually more negative, the time of new or platecurrent,'instead of diminishing'gradually to zero will diminishgradually to a certain point, and then shut oil to zero suddenly. If, onthe other hand,

the angle of lag is greater than 90, then as the controlling directcurrent potential in the grid circuit becomes gradually more positive,the time of fiow of plate current; instead .01 gradually increasing to amaximum conditiom'will increase gradually to a certain point only, fromwhich any further increase of positive direct current potential'willcause the plate current to .fiow during the-entire positive half of thecycle and thus be at" a -maximum. In simple anguage, 1 therefore,

if the angle istoo' small, the tube will close suddenly or with a hump",and if too large, the tube will "bump" wide open.

Itwillnowbeapparentthatfullrangedirect I current control is possibleonly with a fixed alterthis is desired the phase of the grid currentpotential is set lagging sufiiciently less than 90 so that current willpass to the desired extent. Current passing more than this minimum thenmay be made to vary with the direct controlling i potential through theworking range of the motor. With this arrangement the control of curerent fiow of a value less than necessary to keep the motor revolving iseliminated. Likewise it is possible by setting the phaseangle of gridpotential at more than 90 lag to set up a control range within low speedlimits which passes at a single jump without intermediate range to ahigh speed.

' In both cases, however, the phase relation is fixed.

0i course, there may be a supplemental phase shift control dependent onthe direct current control. For example, if the phase were caused tocontinually decrease in lag as the direct current is" increased and thetime of plate current fiow increases, a limited variation of this phaseangle as a supplement to the direct current potential throughout theentire range could be used. A supplemental phase shift control whichalways increasesythe angle of lag when the direct current potential iswell on the negative side and is decreasing and always decreases the.angle of lag when the direct current potential is onthe positive sidemight be employedas supplemental to the direct current control.

The effects of relative changesin magnitude 0 the alternating and directcomponents of the grid voltages will be apparent from an inspection ofthe curves of Figures 8 to 11. The magnitude ofalternating componentmust be considerable in comparison with the amplitude of the criticalgrid voltage curve Ee so that harmonics set up in either the grid orplate circuit by the fiow of current through the rectifier shall have a'negligible effect on the operation of the circuit. In

7 practice, therefore, there is a rather definite minimum of value ofalternating current voltage which will provide smooth operation. Withpositive direct current potential, increase of magnitude of alternatingcurrent potential tends to decrease the time oi. fiowof the platecurrent, and with negative direct current potential increase ofalternating potential magnitude tends to increase riation of alternatingcurrent voltage magnitude may be used as a supplemental control fordirect currentpotential so long as the phaseis undisturbe'd and themagnitude of the alternating potential always ismaintained greater thanapredetermined minimum; 7

liteferring now tothe discontinuous control by' the use of correctaltern'ating currenvphase, it

is r'ios'sible tohave the: rectifier open from the shut-oi! position toa conditionwhere" it passes just sufficient current-to cause rotation'of a motor andthen from this point to' open gradually to full openingso that the speedof the motor vanes'with the direct current, control ofthe circult, there being aco'nstant direct curre t fErid voltagebias'just insuflicient to start the motor.

, This characteristic of sudden discontinuance of smooth control may beuseful in other relations. For example,,it may be desired to have arapid rate of motion'of a motor'to feed work or a tool into closejuxtaposition whereupon the feed may be suddenly slowed to the ratedesired for the machining operation. The rectifier is thus wide open.when there is" no. controlling direct current'on it, but as soon asthiscontrolling div rect current is applied there is immediately a which therectifier passes from gradual control to instantaneous full opening,while the turn-oil!" point means where the rectifier closesinstantaneously. from some partially opencondition.) The operation ofthis discontinuity and its control maybe more readily understood byreference to' Figures 12 t is. Figure 12 illustrates the icon- 7 di'tionwhere thegrid potential curves A, B and C lag the plate potential curveEp by zero degrees, that is, are exactly in phase. The curve Esrepresents the critical value of grid voltage curve as in Figures 3 to10. If the potential of the main open until the. plate current falls tozero and the grid potential becomes less than this critical value; a

Curve A of Figure 12 shows the grid potential at its mostpositive-position. The axis of thiscurve is minus 5volts with respect tothe zero line, that is; there is a. negative direct current potentialcomponent of five voltson the grid circuit. This curve A intersects Eeat very nearly five degrees so that the rectifier opens almost as soon.as the; plate potential becomes positive and remains open during theremainder of thehalf cycle. The curve B is similar to curve A, exceptthat the direct current grid component is minus 20 volts, fifteen volts-more negative than in curve A. The intersection of the curve A with thecurve Eo occurs at alater point in the cycle as a result of thismorenegative bias and the rectifier is open for approximately 145instead of about i70 as in curve A. Curve 0 has to a value ofthirty-sixvolts. ,Thiscurve C is substantially. tangent to the curve B at apointof 9oin thecycle so thatthe rectifier is open about 90. The operation;of the rectifier under the conditions shown .in .12-under the .l'directcurrent control 'i'rom minus ,5. volts to minus thirty-six. voltsissmooth andcontinuous.

Asthe potential becomes more 'negativeithe time of tube-openingdecreases until at the; minus the negativedirect current componentincreased thirty-six volt' cor'idition the, v rectifier is open ninetydegreesduring ev ry cycle. It the direct ."current grid" voltage isfurther decreased, the grid potential curve will-fail to reach thecritical curve and the rectifier will not open at all. 'lShus, therectifierunder the conditions shown will operate smoothly on directcurrent control from one hundred'eighty degrees to ninety degrees andwill then close of! entirely in a sing step from ninety degrees to fulloff.

Fi ure 13 illustrates a similar situation. except that in this case thealternating potential of the grid lags the plate potential by forty-fivedegrees instead of zero. With this alternating potential it is necessaryfor the direct current potential to be positive twenty-nine volts tocause full opening of the rectifier as indicated in In the curve A ofFigure 13. Curve B of minus 7 volts direct potential again illustratesan intermediate position with the rechfier open about 130. Curve C,minus 35 volts is just tangent to the curve Ec and allows the tube toopen for 15 fifty degrees during each cycle. If decreased below this therectifier becomes entirely closed. Thus by increasing the phase lag ofthe grid potential from zero to-45. as shown in Figures 12 and 13, thecontrol range has been extended from that between 180" to 90 in Figure12'to that between 180 and 50 in. Figure 13. It the phase lag wereincreased to 90, the control range would extend from 180 to which is thecondition for full range control. It, on the other hand, the phase ofthe grid lags more than 90, conditions similar to those shown in Figures14 and 15 would exist. Figure 14 shows the phase of the grid lagging by135. Here with a direct current potential component or minus 23 volts,as indicated oncurve C, this curve intersects the critical voltage curveEe at nearly 180, so that it is nearly completely closed. Curve B showsthat with the direct current grid potential at zero, with the samemagnitude or. and lag in the alternating current grid component as incurve C, the critical voltage curve Es is intersected at 120 so that therectifier is open during 55 of the positive plate cycle. The controlfrom the position of curve C to theposltion of curve B is continuous andsmooth and so remains until the position indicated by the curve A isreached with a direct potential component of plus 23 volts and with therectifier open through 100 0! the cycle. In other words, from minus 23volts to plus 23 volts direct current grid potential, the rectifiergradually opens from the oil. position to 100 opening. A furtherincrease of the positive dlrect potential component, however. causes thegrid potential curve to intersect the critical curve E0 at zero degreesso that the rectifier will be opening during the entirepositive hall ofthe plate cycle. Thus the tube will open from 100 to. 180 in a singlestep and will remain wide 55 open as long as thegrld potential has adirect current component of more than plus 23 volts. Figure 15illustrates the same eflects as Figure 14; except that the alternatingcurrent component of the grid potential lags the plate current supooplyby 165 instead of 135. Curve 0 in these figures; with a directcurrent potential component or minus 8 volts holds the rectifier at thepoint of' just turning on. In curve B with apero direct currentcomponent of grid potential the 65 rectifier is open for 20', while incurve A having a direct current potential component 01 plus 9' volts therectifier is open for 35". Any further increase in the positivepotential will cause this 8 1d voltage curve to intersect the criticalgrid 70 voltage curve Fe at the'zero point, causing full pointed out,just 90- lag, while the latter, where 10 smooth control has entirelyvanished, is at 180 lag. It is thus'apparent from these curves that acombination'oi' smooth and sudden change of plate current flow can'beobtained from a continuous direct current potential variation byemploying an alternating current component of suitable magnitude andphase. The point of change from smooth to sudden control is almostentirely a matter of phase relation, while the ratio of change of directcurrent grid voltage to change of output which occurs during the smoothportion'ot the control range may be varied within wide limits by properchoice 01' magnitude oi the alternating grid component.

In Figure 19 is illustrated a motor control for reverse rotation of amotor. This circuit shows reverse field windings 20 and II for the motorarmature l9, passage of current through each reverse field winding beingcontrolled by its own rectifier tube 22 or 23 similar to the diagram 01'Figure 7. At 24 and 25 are the sources of out of phasealternating gridvoltage component, and the variable controlling direct current gridcomponent is connected in at 30 so arranged that as it varies it makesthe grid of one rectifier more positive by the same amount that it makesthe grid of the other more negative. Means by which two direct currentvoltages may be caused to vary equally and oppositely in response tovariations oi a controlling variable are old and well known and per seform no part 01 the present invention. An example 01' such means isshown in the patent to Wold, No. 1,232,879, patented July 10, 1917. Byusing a phase lag from the units and 25 sumciently less than,90 so thatthe rectifiers pass just enough current to turn the motor as soon asthey pass any current at all, and with proper biasing direct current asfrom the units N and i5 and magnitude oi the alterating grid potential,the motor may be controlled smoothly from full speed rotation, in onedirection through stop to full speed rotation in the other direction bymovement of the slider 3| without any range of motion during which themotor is stationary.

Figures 16, 17 and 18 illustrate the fact that the-introduction ofharmonic frequencies in the alternating current component 01 gridpotential will produce eflects comparable with those resulting fromshift of phase 01' the fundamental componentr By a careful choice ofharmonics it is possible to obtain two points of break in theoutputcurve, combining the effects shown in Figures 12 and 13 with thoseof- Plgures 14 and 15. Thus the rectifier-can be made to turn on to apredetermined value at asinsle step, thenopen smoothly to' anotherpredetermined point, and then open at once wide. Referring to thesefigures. it will be noted that the grid control potential waveis nolonger a sine wave, but a complex wave containing harmonics. The curveII shows the rectifier opening initially to a predetermined value, fthen openingsmooth for a further range as'the direct'control currentbecomes positive, and then in a step opening wide. Figure '16shows thesepoints in the lowest and highest grid voltage curves-respectively, atabout 10 and 113. Figure l7 shows the rectifier opening at a single stepfrom closed to a predetermined opening of v 41 degrees, as the directcurrent component of grid potential becomes more positive than minus 80volts; then passing gradually from 41 degrees open to '77 degrees openas the direct current component of grid potentialpasses from minus 80volts to minus 40 volts; when the direct current componentof gridpotential becomes more positive than minus 40, volts, the tube opens ata single step from 77 degreesopen to 151 degrees open,

and then opens gradually from 151 degrees to 180 degrees or full open asthe direct current component goesirom minus 40 volts to zero volts.

Figure 18. shows the derivation of the complex alternating potentialgrid wave oi Figures 16 and 17irom the wave of the same frequency as ap-20 plied to thepiate (dashline 2:) and another wave (dotted liney) oitwo-thirdsithemagnitude and three times the fundamental frequency oi theline l :r. This third harmonic component may be derived from thesaturation current of an iron cored 25 Y reacta'nce coil run at highdensity. Iiaccuracy in this harmonic is desirable it may be obtained bythe use of a band pass filter in 'a manner well known in the art. It ispossible to flnd a combination oi rundam'ental and harmonic grid po- 30tential which will cause almostany desired curve 01'. control tojexist'It is possible to derive many complex wave forms by a propercombination of slturated reactors. condensers, and resistors. es-

7 l .pecially, ii'thc, reactances oi the condensers emplayedaresomewhere nearthe average impedances of the saturated induct-8110081 fati0! and J00. fl'he'mechanismllllmay be a 50 hygrometric elementresponsive to' the moisture l content of the paper after it leaves thedrier, as, iorexample, as shown 'in the Allen Patent No. 1,781g153jNoV1iibe1 11, 1930, and themecha'nism I02 is a source'of direct currentvoltage rectified from the alternating'current mains H0 and respons ivein direct current voltageto the condi tion of the mechanism I00; itsdirectcurrent ter- 1 1 minals being connected to theleads i020 and I02 IIn-tl'ierlillen patent hereinbei'ore mentioned this directi'currentvoltage actuates' thefmoisture'in- 'dicating'. instrument H In thecontrol-mi thestea'rn valve i I00 1 it is commonly desirable-to have;the variational the steam iflow proportional to; the variation "ofmoisture to which themechanismd lllresponds'withma lim- ;ited ranged'-Ii',-': the moisture goes; outside .this lange it then 'becomesdesirable to set "the steam zvalvesto its emeine osmm in order that theniinimuin'fof'sliall'bproduoed, the r'nois- 7 tureco itent )0 which iesmitside oithis'i' range. in words, en meshes isve'idry the steam-shoulibe" j v ii entirely: the sheet-becomes "1%- did .han' esired, the

" ssteanrvalve'whould be 75 amount of steam slightly less than thatpresumsteam should be turned on full and there left as long as there isany moisture greater than this'amount in the .paper. It is apparent thatit is necessary to interpose some mechanism betweenthe measuringapparatus'in panelH. including the mechanisms I02 and 103, and the toobtain this result. This apparatus is indicated as the rectifier controlunit and is shown in panel J. Two available characteristics of thissteam control apparatus in the panel K, in order unit are shown in thelower part of this panel in the diagram.

. Assuming the characteristics of this unit as shown, together with thecharacteristics of measuring and control apparatus, the resultantcharacteristic oi steamflow plotted'against moisture is shown in Figure21. Twocharacteristics are there shown marked L and M. Examining curve YM it will be noted that for moisture of ti or less the steam flow iszeroand that for moistures .01 8%% or more the steam flow is 100%, orinother, words, the steam-valve I00 is wide open. For moistures between 5and 8%% the steam flow varies from 30% to 50%, this variation beingproportional to the" variation of moisture in the sheet as aflecting themechanism I03. 5

Figure 22 shows the circuit or the rectifier control. unit indicated inthe central panel J of Figure whose characteristic is shown in thediagram in the lower portion ot this panel. Re-

ierring to Figure 22, this unit consists of three main elements. Thereis a-source of harmonics alternatingpotential I06, and a source ofdirect current potential M], an of whichare independently variable andwhich are shown in Figure 22 set oil from each other by dotted-outlines.The

harmonics arise from an iron cored inductance .the. input lines through.a resistance condenser combination H2, I I3 'whichshifts the phase 01'the potential applied to this inductance by Since the impedance of theresistance H2 and the condenser H3 is approximately 1 0% of theimpedance of the inductance Hi, the current through the inductance IIIis about 10% or the total in this circuit so that the current in theinductance iH andits wave form have substan* tially no effect on thetotal, In serieswith the inductance I i land between itand one end ofthe resistance H2 is a current transformerfi li .01

l to. 10 ratio across which is connected a 750ohm f resistance H5. Thisis'equivalentto connecting a 7.5 ohm resistance between the inductance Hiand the resistance H2. Since'the-impedance oi the inductance i H isapproximately IOOOohms,

the 7.5 ohm series resistance has a negligible eilect; However,thejpotentialilacross the 750 ohm resistance Li i5 will vary exactly asthe current through the inductance Hi. ItJis-,a ;wel1

knowniactthat the magnetizing current" in inductanceacrosswhichisimpressed a sine wave electro niotivefforce contains'aconsiderable portion ofha'rmonics. T v i I Figure 2'3 indicated a'typical/3:11 :curve .ior a standard gr de of laminated; ironfand inFigure-24 is indicated-the; ma net zingmile Ian which would flow throughan inductance having a core of this iron on which there is impressed by45. In other words, the voltage E shown in a sine wave electro-motiveforce E. It will noted that the current In! lags the voltage E byapproximately 45 and'that this currentcontains a considerable proportionof harmonic values.

' Assuming thiscurrent In flows through'the sistor m. Since the resistorin is connected across the line in series with a capacity I ii of 25microfarads having an impedance of approximately 100 ohms, the voltagein the resistance .I I2 will lead the voltage impressed from the lineI'I0 Figure 24 leads the impressed voltage from the line by 45". Sincethe current In lags the voltage E shown by 45", this current In then isin phase with the applied voltaged'romthe line and the voltage acrossthe 750 ohm resistor I llresulting from the current fiow Ira through thel to 10 current transformer III will likewise be in phase with theimpressed potential. This voltage is shown as E of Figures 25 and 26.This voltage E0 is the sum of a certain amount of fundamental plusvarious harmonics.

The second element I" of the control unit consists of a transformer litand resistor III which produces a sine wave of alternating potential inphasewith and exactlyproportiona'l to the input potentialfromthe line III. This alternating potential is added to the fundamental plus harmonicpotential derived from the harmonic source I05. This potential isvariable by varying the tap III on the 1000 ohm potentiometer Ill. Twovalues of this potential are shown- ,added to the harmonic potentials inFigures 25 and 20. and E4 and are subtracted from the E of'these figuresgiving potentials E1 and Es.

The third section Illlof the control. unit in panel J consists of asource of variable direct current potential shown as a battery I20 and apotentiometer I2l, all three sections being'connected in seriesand Inseries with the controlling direct current through leads I020 and I02Ifrom the external source and applied asat I25 and I20 between therectifier grid I21 and the cathode I28 (Figure 20) which rectifiercontrols the positionof the valve actuator I00 shown as a solenoid foropening the gravity-closed valve I00.

Figure 27 indicates the effect of the alternating potential suppliedfrom the harmonic and fun-.

damental source upon the rectifier. As has been previously noted, thesecharacteristics are in phase with the potential applied from the mainsIII which is likewise applied to ,the plate to cathode circuit of therectifier. As will be apparent upon inspection, as the zero line of thealternating potentials varies up and down due to the direct current of.the control unit plus the the tube iseither all oil or at least 25%open. As

controlling direct current cm'vcs E1 or E; (Figs;

26 and 2'!) move up anddown, intersecting curve E: which represents thecritical value of rectifier cut-off. Curve E: in the position as shownlies entirely below the curve Es and hence the tube remains closed. Asthe zero line and with it E: move up due to more positive direct currentpotential the peak of E1 intersects the curve B: at about 25% full tubeopening. In other words,

the zero line continues to rise, the curve Ea intersects the curve Ec ata constantly earlier point until the tube is about. 50% open at whichpoint These potentials aredesignated as E: I

- the rising portion of curve E: at 100% full opening intersects thecurve Ec at 0 of the cycle causing the tribe to .open full. In otherwords, the

tube-opensfgradually from 30% to50% andthen. in a single motion from 50%to 100%. This isv shown diagrammatically in Figure 28, in the fcurveri.Similarly the action of thecharacteristic -El ,1s shown in curve njinthis figure. As

" will noted; Figure 28 is the same as the lower diagram'of panel J ofFigure 20, except .that the scale of direct current volts. has beenaltered. It is apparentthat by varying the direct current potential ofthe'rectifler control unit,v the whole curve, F,igure 28, may be'movedup or down at will.', Figure 28; represents the action of thealternatinglcurren't components supplied by the rectifier control unit.The diagram in panel J, Figure 21, indicates the control as a'whole withalterhating plus direct potential components.

It is apparent that by varying the fundamental components of thealternating current control any curve value between that shown at L andM of Figure 21 within the limits shown can be produced and that byvarying this value outside the limits shown other curves will result.Furthermore, by increasing or decreasing the magnitude by bothfundamental and harmonic components the ratio-of moisture to steamfiowwithin the -smooth variation section can be varied 'at' will. By varyingthe direct current potential of the rectifier control unit, the entirecharacteristic of Figure 21 may be moved to the right or left, theaction occurring within any desired limits of moisture. Furthermore'byvarying the capacity of the condenser II! of the harmonic'source ofFigure 22,-the harmonic characteristic may be displaced to the right orleft thereby varying the minimum amount of steam fiow up or down as maybe desired. These variations indicate roughly a few of thepossibilities. By suitable combinations of resistors, iron inductanc'es,and capacity almost any desired wave form or wave forms can be 'producedresulting in almost any desired response characteristic of the rectifiercontrol unit and thereby resulting in any desired relationship betweenthe controlling direct current potential and the response of therectifier and giving any desired relation between sheet moisture andsteam flow. I

From the foregoing description of the method of operation andfundamental circuits showingits application, it should be evident tothose skilled in the. art that various other changes and modificationsmight be made without departing from the spirit or s'cope'of thisinvention as defined by the appended claims.

I claim: I

1. The method of operatively controlling electrical power passed by anionic valve having a plurality of electrodes and voltage-responsivemeans for controlling the time of commencement of current fiow throughsaid valve, which comprises impressing a cyclic voltage difi'erentialbetween said electrodes periodicallyreaching a value to stop the powerfiow once started, impressing on said control means a cyclic voltage ofthe 2. The method of operatively controlling elecricai power passed byan ionic valve having a plurality'ot electrodes and voltage-responsivepressing on 'said cyclic voltage a voltage having a direct currentcomponent, and varying said component to control the passage of saidelectricalpower.

3. The method of controlling a condition influenced by the electricalpower passed by an ionic valve having a plurality of electrodes andvoltage- 20' responsive means ior controlling the timeof commencement ofcurrent now through the valve,

which comprises impressing'a cyclic voltage differential between saidelectrodes periodically reaching a value to stop the power flow once Pstarted, impressing on said control means a cyclic voltage having apredetermined program of variation chosen in accordance with, theparticular needs of the condition to be controlled, and varying theposition of the axis of said cyclic voltage 30 to control the passage ofelectrical power.

4. The method of controlling a condition influenced by the electricalpower passed by an ionic valve having a plurality of electrodesandvoltage-responsivemeans for controlling the time of commencement ofcurrent flow through the valve, which comprises impressing a cyclicvoltage differential between said electrodes periodically reachingavalue to stop the power flow once startedyimpressing on said controlmeans a cyclic voltage having a predetermined program of variationchosenin accordance with the particular needstof theconditionto becontrolled, and varyg a continuous direct current. component of idcyclic voltage to, control. the electrical power a ssedlby-fsaid-valve.a

* 5. 'ill' e method of cont'r lling acOnditi n influen I d-byeIectricalpOwer passedby anjlonic valve haJlir g -a plurality; ofelectrodes and voltageesponsiye means for cont; oiling the time of com-I so mencement of. current flow through said valve,

.which comprises impressinga cyclicvoltage difjferential. "between saidelectrodes periodically reaching-a 'value-'tostop the power now oncestart ed, impressing on" said control means a cyclic v5 control,potential {made up from a plurality .of

mn t ot ue hermeti l r at H toisaid voltage "differential,predetermining the ipha'sefland; magnitude of said components'inaccordance with t he; program of variationdesired,

and varying th e positiori'oi the axis of said cyclic voltage ,to .co'ntrolthe passage of'electric power.

' 6; The methodi' of op'erativ'ely controlling the output of anarc-rectifierhaving a'separate po-' tential control-of. ignition time,and having alter- 65 nating' potential difierences impressed thereacrossthrough a load. which comprises impressing alternating potential of thesame frequency I as said potential differences and a direct currentpotential on said' time control, controlling said 70 direct currentpotential, and controlling the phase relation of said alternatingignition control potential to said potential difierences to determinethe *e'xtent and position of discontinuity of smooth,

control by direct current ignition time voltage 75 variations.

of operatively controlling the '1. The method output of an arc rectifierhaving aseparate po- 7 across through a load, which comprises impress:

potential ing alternating potential of the same frequency as saidpotential diil'erences and a direct current potential on the timecontrol, controlling said direct current potential, and controlling therelative magnitudes of the alternating current and direct current timecontrol voltage components to determine the ratio oi variations inoutput of said rectifier to variations in controlling direct currenttime control voltage.

,8. The method of operatively controlling the output of an arc rectifiertube having a separate control of ignition time, and having alternatingdifierences impressed thereacross through a load, which comprisesimpressing alternating potential of the same frequency as said potentialdifferences and a direct current potential on the time control,controlling said direct, cur- 7 rent potential, controlling the relativemagnitudes of the alternating current and direct current ignition timecontrol voltage components to determine the ratio of variations inoutput of said rectifier to variations in controlling direct currenttime control voltage, and controlling the phase relation of saidalternating control potential tosaid potentiaLdiiferences to determinethe extent and position of smooth output control by the direct currenttime control voltage variation.

-. 9. The method of smoothly controlling the outsaid rectifieralternating potential of the same' frequencies as the input powerandlagging. the same by 90 and acontrolling direct current potential foroperation for said entire range, and

controlling the value of said direct current ignie tion controlpotential.

10,.The method o1 operativelycontrolling electrical power passed by anionic valve having-a plurality of v electrodes and voltage responsivemeans for controlling the time of commencement havinga direct currentcomponent, selecting a phase relationship between said cyclic voltageand said voltage differential sufliciently. difierent grog; 90 toproduce a discontinuity in the smooth control of said valve by variationof said component at the desiredpoint in'the control, and

varying said component to control the passage of electric power.

, oi current flow through said valve, which cor'nj prises impressing acyclic voltage differential between Saidelectrodes periodically reachinga 11. Theme thod of controlling the output Oran arc rectifier havingaiseparate potential control of ignition time, to provide apredetermined minimum flow greater than zero, which comprises impressingon the ignition control a direct current voltage and an alternatingcurrent voltage component of the same frequency as the rectifier input,said alternating current voltage component lagging the input phase by anamount sufllciently less than 90 to produce the desired minimum currentflow, and varying said direct current voltage.

12. The method of controlling the output of an impressing on theignition control a direct current controlling voltage and an alternatingcurrent component of the same frequency as the rectifier input, saidalternating current ignition control voltage componentilagging the inputphase by an amount sufliciently greater than to produce full opening ofsaid rectifier at the desired point, and varying the direct currentcomponent of ignition control voltage.

13. The method of controlling the output of an are rectifier withseparate potential control of ignition time for smooth control within arange less than from no current to full current passage, which comprisesimpressing on the ignition control of said rectifier a controllingdirect current potential and an alternating current potential of thesame basic frequency as the input power, and having the higherharmonics, selecting such harmonics and the phase relation between thebasic wave of said ignition control alternating current voltagecomponent to said input to produce the desired departure from smoothfull range control, and varying the'amount of said controlling directcurrent ignition control voltage.

14. The method of controlling a motor fed from an arc rectifier withseparate control of ignition time, which comprises impressing on theignition control of said rectifier a controlling variable direct currentpotential and an alternating current potential of the same frequency assaid rectifier input and sufiiciently less than 90 lagging in phase tocause smooth control of rectifier output through said motor byvariations of controlling direct current potential to begin with flow ofouput current substantially just sufiicient to start said motor.

15. The method of controlling a reversible motor fed from are rectifiersfor opposite direccontrol of ignition time, which comprises impressingon the ignition controls of said rectifier controlling variable directcurrent potentials the one increasing while the other correspondinglydecreases and alternating current potential of the same frequency as theinput to said rectifiers and suificientiy less than 90"lagging in phaseto cause smooth control of rectifier output to said motor by variationof said controlling direct current potential to begin with fiow ofcurrent substantially just suiiicient to start said motor, wherebysmooth control of said motor from operation in one direction throughstop and operation in the reverse direction is produced by continuousvariation of said direct current control potentials.

16. The method of controlling a motor fed from an arc rectifier withseparate control of ignition time, comprising impressing on the ignitioncontrol of said rectifier a controlling variable direct currentpotential and an alternating current potential of the same-frequency asthe input to said rectifier and sufliciently more than 90-lagging inphase to cause smooth control of motor speed only throughout a range apredetermined amount less than that caused by a maximum opening of saidrectifier to current flow. whereby at the upper limit of said range themotor has suddenly impressed thereon the maximum current fiow which saidrectifier can pass. a

17. The method of controlling the output of an arc rectifier withseparate potential control of ignition time for smooth control within arange less than from no current to full current passage, which comprisesimpressing on the ignition control of said rectifier a variablecontrolling direct current potential and an alternating potential of thesame frequency as the input to said rectifier but containing-harmonicsand in correct phase relation to said input to cause sudden closing oropening of said rectifier to a predetermined lower partial opening limitto be produced by variation of said direct current potential adjacent toa relatively low point, sudden full opening or closing from apredetermined higher partial open v limit by variation of saiddirectcurrent potential adjacent to a relatively high point and smoothcontrol of the extent of opening ofsaid' rectifier between said limitswith smooth variation of con-v trolling direct current potential betweensaid relatively low and high points.

18. A power circuit comprising an arc rectifier having a separatepotential control or ignition time, aload in the rectifier output; meansfor impressing alternating potential across said recti fier, means forimpressing a controlling'variable direct current potential on saidignition control, and means for impressing an alternating currentpotential on said ignition control of the same fundamental frequency as.the input to said rectifier but with higher harmonics and so related tothe phase of said input potential as to cause smooth variations incontrolling direct current ignition control voltage throughout a controlrange having its lower limit at a predetermined minimum output currentfiow greater than zero and its upper limit at apredeterminedmaximum lessthan that of full rectifier opening;

19. A power circuit comprising anarc rectifier I having a separatepotential control of ignition time, a load in the output circuitcomprising a motor, means for impressing alternating potential acrosssaid rectifier; means for impressing controlling variable direct currentpotential on said ignition control, and means for impressing analternating current potential on said'ig'nition control ,of the -samefundamental frequency as said rectifier input potential but with1.hlgher harmonicszso related toithe phase of said inputpotentialiasjocause smooth variations in controlling direct current'ignition control I voltage throughout; a control range having its lowerlimit ata predetermined minimum output current flow greater than zeroand substantially just sufficient to start rotation of said motor auditsupper limit at a predetermined maximumzless than that of fullrectifieropening and where saidmotoris rotating at aspeed lessrthanmaximum corresponding to .fullrectifier opening.

' wILFaED H. nown v

